1. Field of the Invention
The present invention relates to a telephone switching system, and more particularly to a telephone switching system applicable to a PBX (Private Branch exchange) accommodating, e.g. units adaptive to an existing communications network such as a public switched telephone network and a broadband network.
2. Description of the Background Art
Today, the rapid growth of broadband communications has not only made transmission equipment on the network side adaptive to broadband services, but also promoted the replacement of ordinary subscriber lines with broadband circuits such as ADSL (Asymmetric Digital Subscriber Line) or optical fibers. In parallel with such spread of broadband networks, or broadband IP (Internet Protocol) networks, broadband CODECs (COder-DECoders) are used so that communications units and terminals such as IP telephone sets are spreading for thereby enhancing communications quality.
A conventional switching network has, in many cases, a particular kind of CODEC. In other words, a particular coding rule is assigned to each interface. Such a switching network was therefore, in this sense, problematic little. However, considering the current trend toward the mixture of a conventional network such as a public switched telephone network with a modern broadband network, it is necessary to select a particular kind of CODEC for each call. For example, a conventional circuit switching system or a conventional PBX uses a coding rule that basically accords to ITU-T (International Telecommunication Union-Telecommunication standardization sector) Recommendation G.711 (G.711 coding rule hereinafter). However, when the circuit switching system or the PBX accommodates a broadband interface according to ITU-T Recommendation G.722 (G.722 coding rule hereinafter), it is necessary to match its CODEC to the other station to be connected. More specifically, communications must be held in a G.722 or broadband mode if the other station connected is a broadband station using the G.722 coding rule, or be in a G.711 mode if it accords to the G.711 coding rule.
Now, the frequency band available with a broadband IP network is basically not limited, and therefore promotes services using broadband audio signals. More specifically, human voices are originally not limited to the frequency band below 4 kHz inclusive, but include harmonics components, so that broadband communications are more desirable in establishing conversation that sounds more natural or less wearing.
IP telephone systems, for example, can implement broadband communications. More specifically, when the G.722 coding rule is used in combination with, e.g. H.323, MGCP (Media Gateway Control Protocol), SIP (Session Initiation Protocol) or similar protocol standardized by the ITU-T, the frequency band for communications can be broadened if CODEC negotiation is executed. However, the following problem would arise when an IP telephone set is accommodated in a conventional PBX based on a time switch.
Generally, the coding rule of a conventional PBX is fixed to the G.711 or similar coding rule not adaptive to broadband applications. If an IP telephone set adaptive to broadband communications is accommodated in such a PBX, then the IP telephone set must be designed to negotiate a coding rule with a central control module included in the PBX for controlling call switching processing. However, it would be impractical for the central control module, which inherently bears a heavy load, to perform negotiation on the coding rule as well.